Multiple sheet business form assemblies which can be used as mailers are typically sealed envelopes which contain additional sheets therein on which information can be printed by impact printing processes. These mailers are frequently processed by computer-aided automated systems. The ability to print information selectively both on the inside and on the outside of the mailer simultaneously eliminates time-consuming separate printing of the sheets and envelopes, and manual stuffing, sealing, and stamping of individual envelopes. Mailers of this type are described in U.S. Pat. Nos. 3,777,971, 3,830,141, 3,988,971, 4,081,127, 4,095,965, 4,425,386, and 4,729,506.
In producing this type of mailer, confidential information, such as billing information, appears only on the inner layers of the mailer, while other information, such as the address, must appear on the outside. A variety of techniques have been proposed to accomplish this result.
One prior method involves printing all of the information on the outside of the mailer and onto one or more interior sheets The confidential information, which is to appear only on the inner layers, is then obscured by overprinting the confidential information on the exterior surface of the mailer with enough printed characters to make it unreadable. However, envelopes printed in this fashion have an undesirable appearance and require additional printing steps.
Another prior method involves the use of spot coatings of carbon ink on the bottom side of a top record sheet positioned over the mailer. Using this method, the address information can be printed on the front of the envelope by positioning it under the carbon spot, while confidential information can be printed onto the interior sheets. However, this system requires the use of expensive carbonizing bond paper to prevent penetration of the carbon ink. In addition, the top record sheet is typically thrown away because the information recorded there is usually stored on computer.
Another type of mailer uses a carbon ink spot coated carbon tissue which is placed between the top record sheet and the mailer. This eliminates the use of the expensive carbonizing bond paper, but the carbon tissue must be removed and thrown away prior to mailing.
Carbon ink spot coated carbonizing bond paper and carbon tissue sheets have sometimes been used inside the mailer. However, as discussed previously, these systems are expensive and, in addition, can cause smudging of the image receiving sheet.
Another system, is disclosed in U.S. Pat. No. 4,172,605, which uses a chemically reactive ribbon for a typewriter or a computer printer. The ribbon is coated with a solution which reacts with the coating on portions of the mailer to form images. This system requires direct contact between the ribbon and the mailer so that the chemical reaction can take place. In addition, the ribbons, which are relatively expensive, must be replaced frequently.
Still another system is disclosed in U.S. Pat. No. 4,425,386 to Chang. The mailer uses an autogenous, or self-contained, coating over part of the top sheet. The impact of the printer or typewriter causes the image to develop in the coated area. Other information printed at the same time but outside the coated area will not appear on the top sheet. The interior sheets of the mailer may have self-contained coatings or other printing systems so that the other information will appear on the appropriate sheets.
While the mailer described in the Chang patent solves some of the problems associated with mailers, it suffers from some serious drawbacks. The Chang patent teaches the use of a typical self-contained coating, such as that described in U.S. Pat. No. 4,010,292. Such a self-contained coating contains an encapsulated color former dissolved in oil and a dispersed color developer, such as phenolic resin or acid clay.
However, such self-contained coatings suffer from a number of, disadvantages. One problem with typical self-contained coatings is that dispersions of color developer usually must be applied as low solids content dispersions due to the poor rheology (i.e., high viscosity) of commercially available phenolic dispersions. The low solids level results in higher drying requirements, weak image development, and sheet distortion upon drying. Poor rheology also limits the method of applying these coatings to air knives or other full coat applicators, effectively preventing their use as spot coatings. In some cases, the addition of high levels of binder to the coating may permit it to be printed as a spot coating, but this results in poor image development. Background coloration is a major problem with conventional self-contained coatings. The existence of free color former in the coating remaining after the encapsulation process causes premature coloration of the background when it reacts with the dispersed color developer which is also present. Additional background coloration can be caused by the tendency of phenolic resin dispersions to yellow when exposed to air.
Another problem with typical self-contained coatings is the waste of expensive color forming dyes. Much of the dissolved dye remains in the capsules and is not available for reaction with the color developer when the capsules are ruptured. This requires the use of higher coating weights to achieve a sufficient surface concentration of dye and results in a significant waste of dye.
In its move toward automation, the U.S. Postal Service has issued specifications related to machine readability of mail. See Publication 25, United States Postal Service (1988). The specifications indicate that a print reflectance difference of at least 30% in the red portion of the optical spectrum is necessary for satisfactory reading of the post office bar codes. Print reflectance difference is defined as the difference between the reflectance of the background, i.e., the envelope, and the reflectance of the ink, multiplied by 100.
A mailer using a self-contained carbonless imaging system which passes this specification would be very desirable. However, because of the problems associated with the use of traditional self-contained coatings, prior art mailers have not been able to meet the postal specification because of weak image development.
Similar problems exist in producing covert (i.e., initially colorless) images on a security document or safety paper, such as checks and other negotiable instruments and certificates indicating title or authenticity, using conventional self-contained coatings. Some covert images are rub or abrasion sensitive so that if alteration of a document is attempted, such as erasure of a dollar amount on a check, the pressure causes the encapsulated dye to be released to react with the color developer to produce a colored indication of the attempted alteration.
However, premature coloration problems due to free color former present in the coating after encapsulation may result in a genuine document being dishonored as it may appear that the document has been altered. Further, yellowing of the dispersed color developer as it is exposed to air may also lead to confusion. The need to coat the document with a low solids coating composition leads to excessive drying requirements and potential cockling of the document substrate. Finally the use of encapsulated dyestuffs in solvents produces weak colored images, or alternatively, if higher coat weights are used to compensate for the weak color, results in higher costs because of the amount of dye required.
Other covert images are solvent sensitive. Commercially available solvent sensitive security paper is usually made by dispersing small particles of a solvent soluble, but water insoluble, dye into the headbox at the paper mill. As the paper is made, the solvent dye particles become entwined with the paper fibers and are virtually invisible. When an alteration is attempted by using a solvent which is capable of solvating the dye particle, the dye dissolves and creates a visible stain or speckle on the paper.
The appearance of these speckles is intended to alert the clerk or cashier that an attempted alteration has occurred and the document should be voided. However, in order for this system to be effective, the clerk must be aware that the speckles indicate attempted alteration and that they are not simply part of the document background. In addition, many of the solvent dyes used in the manufacture of the paper are not stable to ultraviolet light exposure. They can be faded from visibility by exposure to ultraviolet light either before or after solution.
Therefore, there is still a need in the art for multiple sheet business forms or mailers which can be printed with information on both the exterior and interior surfaces thereof. Further, there remains a need for a mailer which can produce imaged information which passes the postal specification for machine readability. There is also a need for compositions, self-contained as well as others, which can be printed by conventional printing equipment as a high solids content dispersions. Yet further, there remains a need in the art for compositions which can be printed as covert images on a security document or safety paper and which are not subject to the many shortcomings of the prior art compositions.